Hi there,
Echoing William about not wanting to step on another's business, but on-page content creation is virtually indistinguishable from the process of creating internal links, in my opinion as an SEO and as someone with a background in writing / publishing. They're generally worked into copy naturally, then site structure (which should have been covered in the site audit) would cover issues like navigational linking.
I worked in the sales section of my former agency for the last year I was there (former SEO consultant for them, now back to freelancing) and we certainly never broke anything down like this.
What I did find, however, was that larger clients (think corporate giants) want break-downs of everything cost-wise when proposing work. They want to know what the individual cost of writing pages, removing links, even the cost of a consultant's time to monitor tools on a monthly basis. However, adding invoices for things after the fact was clearly a big no-no. The retainer or full cost was stated up front (e.g. $10,000 per month) and then a breakdown was shown, rather than work beginning and then me saying, "Oh, so it's going to be another $2k for X, Y and Z."
I can see something like the cost of rewiring internal links being included in a breakdown like this, but if I were writing a quote for re-working the on-page elements of a website, it would certainly not come with an extra invoice for internal links.
$20,000 for an initial phase of work before link dev is understandable if you were on a monthly retainer of $10,000 and a massive amount of initial work was put into the campaign - this is the sort of fee I used to be working with when at an agency. But with that fee came a culture of all-inclusiveness. Everything from site audits to link audits and take-downs, social media audit, 12 - 24 month strategy, staff training, tool access, etc. came included.
Short version: I'd not be a fan of including these costs after work has started, rather than the company saying: "This is going to cost you $40,000 over six months" and showing the breakdown of where that money is spent. I also personally find the cost of placing internal links to be a natural part of both the site audit and copywriting, so am confused about why that's separate.